U.S. patent number 6,958,099 [Application Number 10/419,967] was granted by the patent office on 2005-10-25 for high toughness steel material and method of producing steel pipes using same.
This patent grant is currently assigned to Sumitomo Metal Industries, Ltd.. Invention is credited to Toshiharu Abe, Kaori Kawano, Shigeru Nakamura, Tomohiko Omura.
United States Patent |
6,958,099 |
Nakamura , et al. |
October 25, 2005 |
High toughness steel material and method of producing steel pipes
using same
Abstract
A steel material and a steel pipe made by using the same are
provided which are to be used in severe oil well environments. Such
a highly tough oil well steel pipe can be produced by rolling the
base material, quenching the rolling product from the austenite
region and tempering the same so that the relationship between the
content of Mo [Mo] in the carbides precipitated at austenite grain
boundaries and the austenite grain size (according to ASTM E 112)
can be defined by the formula (a) given below. In this manner,
steel pipes suited for use even under oil well environments
becoming more and more severe can be produced while satisfying the
requirements that the cost should be rationalized, the productivity
improved and energy saved.
Inventors: |
Nakamura; Shigeru (Wakayama,
JP), Kawano; Kaori (Neyagawa, JP), Omura;
Tomohiko (Kishiwada, JP), Abe; Toshiharu (Osaka,
JP) |
Assignee: |
Sumitomo Metal Industries, Ltd.
(Osaka, JP)
|
Family
ID: |
19066807 |
Appl.
No.: |
10/419,967 |
Filed: |
April 22, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCTJP0110920 |
Dec 12, 2001 |
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Foreign Application Priority Data
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Aug 2, 2001 [JP] |
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2001-235349 |
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Current U.S.
Class: |
148/334; 148/328;
148/593; 148/663; 148/654 |
Current CPC
Class: |
C21D
8/10 (20130101); C22C 38/002 (20130101); C22C
38/28 (20130101); C22C 38/22 (20130101); C22C
38/04 (20130101) |
Current International
Class: |
C22C
38/00 (20060101); C22C 38/28 (20060101); C22C
38/22 (20060101); C21D 8/10 (20060101); C22C
038/12 (); C22C 038/22 (); C21D 008/10 () |
Field of
Search: |
;148/654,663,593,334,328 |
Foreign Patent Documents
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58-224116 |
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Dec 1983 |
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JP |
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05-271772 |
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Oct 1993 |
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JP |
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06-172858 |
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Jun 1994 |
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JP |
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2000-017389 |
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Jan 2000 |
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JP |
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2000-178682 |
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Jun 2000 |
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JP |
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2000-256783 |
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Sep 2000 |
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JP |
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2000-297344 |
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Oct 2000 |
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JP |
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2001-073086 |
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Mar 2001 |
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JP |
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Primary Examiner: Yee; Deborah
Attorney, Agent or Firm: Clark & Brody
Parent Case Text
This application is a continuation of International Patent
Application No. PCT/JP01/10920 filed Dec. 12 2001. This PCT
application was not in English as published under PCT Article
21(2).
Claims
What is claimed is:
1. A steel material having high toughness comprising molybdenum and
carbides precipitated at austenite grain boundaries, a content of
Mo [Mo] in the carbides satisfying the formula (a) given below:
where G is the austenite grain size number according to ASTM E
112.
2. A steel material having high toughness comprising by mass %, C:
0.17-0.32%, Si: 0.1-0.5%, Mn: 0.30-2.0%, P: not more than 0.030%,
S: not more than 0.010%, Cr: 0.10-1.50%, Mo: 0.01-0.80%, sol. Al:
0.001-0.100%, B: 0.0001-0.0020% and N: not more than 0.0070%, the
balance being Fe and unavoidable impurities, the steel including
carbides precipitated at austenite grain boundaries; and a content
of Mo [Mo] in the carbides satisfying the formula (a) given
below:
where G is the austenite grain size number according to ASTM E
112.
3. A steel material having high toughness comprising by mass %, C:
0.17-0.32%, Si: 0.1-0.5%, Mn: 0.30-2.0%, P: not more than 0.030%,
S: not more than 0.010%, Cr: 0.10-1.50%, Mo: 0.01-0.80%, sol. Al:
0.001-0.100%, B: 0.0001-0.0020% and N: not more than 0.0070% and
further containing one or more of Ti: 0.005-0.04%, Nb: 0.005-0.04%
and V: 0.03-0.30%, the balance being Fe and unavoidable impurities,
the steel including carbides precipitated at austenite grain
boundaries; and a content of Mo [Mo] in the carbides satisfying the
formula (a) given below:
where G is the austenite grain size number according to ASTM E
112.
4. A steel material having high toughness comprising by mass %, C:
0.20-0.28%, Si: 0.1-0.5%, Mn: 0.35-1.4%, P: not more than 0.015%,
S: not more than 0.005%, Cr: 0.15-1.20%, Mo: 0.10-0.80%, sol. Al:
0.001-0.050%, B: 0.0001-0.0020% and N: not more than 0.0070% and
further containing one or more of Ti: 0.005-0.04%, Nb: 0.005-0.04%
and V: 0,03-0.30%, the balance being Fe and unavoidable impurities,
the steel including carbides precipitated at austenite grain
boundaries; and a content of Mo [Mo] in the carbides satisfying the
formula (a) given below:
where G is the austenite grain size number according to ASTM E
112.
5. A method of producing highly tough steel pipes for oil wells,
comprising hot rolling a steel which contains, by mass %, C:
0.17-0.32%, Si: 0.1-0.5%, Mn: 0.30-2.0%, P: not more than 0.030%,
S: not more than 0.010%, Cr: 0.10-1.50%, Mo: 0.01-0.80%, sol. Al:
0.001-0.100%, B: 0.0001-0.0020% and N: not more than 0.0070%, the
balance being Fe and unavoidable impurities, the steel including
carbides precipitated at austenite grain boundaries; and quenching
the hot rolled product from the austenite region, wherein, after
the subsequent tempering, a content of Mo [Mo] in the carbides
satisfies the formula (a) given below:
where G is the austenite grain size number according to ASTM E
112.
6. A method of producing highly tough steel pipes for oil wells,
comprising hot rolling a steel which contains, by mass %, C:
0.17-0.32%, Si: 0.1-0.5%, Mn: 0.30-2.0%, P: not more than 0.030%,
S: not more than 0.010%, Cr: 0.10-1.50%, Mo: 0.01-0.80%, sol. Al:
0.001-0.100%, B: 0.0001-0.0020% and N: not more than 0.0070% and
further containing one or more of Ti: 0.005-0.04%, Nb: 0.005-0.04%
and V: 0.03-0.30%, the balance being Fe and unavoidable impurities,
the steel including carbides precipitated at austenite grain
boundaries; and quenching the hot rolled product from the austenite
region, wherein, after the subsequent tempering, a content of Mo
[Mo] in the carbides satisfies the formula (a) given below:
where G is the austenite grain size number according to ASTM E
112.
7. A method of producing highly tough steel pipes for oil wells,
comprising hot rolling a steel which contains, by mass %, C:
0.20-0.28%, Si: 0.1-0.5%, Mn: 0.35-1.4%, P: not more than 0.015%,
S: not more than 0.005%, Cr: 0.15-1.20%, Mo: 0.10-0.80%, sol. Al:
0.001-0.050%, B: 0.0001-0.0020% and N: not more than 0.0070% and
further containing one or more of Ti: 0.005-0.04%, Nb: 0.005-0.04%
and V: 0.03-0.30%, the balance being Fe and unavoidable impurities,
the steel including carbides precipitated at austenite grain
boundaries; and quenching the hot rolled product from the austenite
region, wherein, after the subsequent tempering, a content of Mo
[Mo] in the carbides satisfies the formula (a) given below:
where G is the austenite grain size number according to ASTM E 112.
Description
TECHNICAL FIELD
This invention relates to a steel material having a high level of
toughness and suited for use in producing steel pipes to be used
under severe conditions in oil well environments and to a method of
producing steel pipes for oil wells using the same while
rationalizing the cost, improving the productivity and, further,
saving energy.
BACKGROUND ART
In recent years, the oil well drilling environment has become more
and more severe, and steel pipes for oil wells used on each spot
are now exposed to an oil well drilling environment containing
carbon dioxide and the like in addition to the increasing depth of
oil wells. The steel material to be used in producing such steel
pipes is required to have strength and toughness characteristics.
In particular, oil wells to be developed in the future are expected
to be ones having a greater depth or horizontal ones and,
therefore, the steel pipes to be used are required to have still
higher strength and toughness performance characteristics than the
levels so far required.
To cope with these requirements, the art has endeavored to produce
high performance steel pipes by reducing the size of austenite
grains in the steel material or by adding an expensive additive
element or elements to thereby improve the hardenabihty. From such
a viewpoint, Japanese Patent No. 2672441, for instance, proposes a
method of producing seamless steel pipes characterized by high
strength and high toughness.
According to the production method proposed in the above-cited
patent specification, the austenite grain size is reduced to ASTM
No. 9 or finer to thereby secure excellent resistance to sulfide
stress corrosion cracking (SSCC resistance) as well as high
strength and toughness performance characteristics.
Thus, the production method proposed in the above patent
specification is intended to give steel species having high
toughness and employs the so far known technique of reducing the
size of austenite grains and, therefore, it is expected that the
reduction in size of austenite grains will cause deterioration in
hardenability. When the hardenability of a steel species becomes
poor, the toughness and corrosion resistance will deteriorate. For
preventing the hardenability of steel from deteriorating, it is
generally necessary to add a large amount of such an expensive
element or elements as Mo.
Furthermore, the production method proposed in the above-cited
patent specification presupposes that direct quenching or in-line
heat treatment be performed directly from the heated state after
rolling, which is then followed by tempering. Therefore, the method
requires strict control of rolling conditions and, in this respect,
it is unsatisfactory for the cost rationalization and production
efficiency viewpoint. The method still has the problem that the
productivity improvement, energy saving and cost reduction
currently required in the production of steel pipes for oil wells
cannot be accomplished.
On the other hand, methods of producing steel pipes for oil wells
capable of showing good performance characteristics in oil well
environments even when the size of austenite grains is relatively
coarse have been proposed. Since intragranular cracking serves as
the origin of breakage with the increasing strength of steel,
Japanese Patent Application Laid-open No. S58-224116, for instance,
proposes a method of producing seamless steel pipes excellent in
sulfide stress cracking resistance which comprises reducing the
contents of P, S and Mn, adding Mo and Nb, and controlling the
austenite grain size within the range of 4 to 8.5.
Further, Japanese Patent No. 2579094 proposes a method of producing
oil well steel pipes having high strength and excellent sulfide
stress corrosion cracking resistance which comprises adjusting the
steel composition and hot rolling conditions to thereby adjust the
austenite grain size to 6.3 to 7.3.
However, any of the methods so far proposed does not mention
anything about the securing of toughness required of steel pipes
for oil wells and cannot be employed as a method of producing oil
well steel pipes having both high strength and high toughness.
Meanwhile, it is known that, for securing the toughness of steel
materials, it is effective to strengthen the austenite grain
boundaries themselves in place of reducing the austenite grain
size. As a means therefor, a method is known which comprises
controlling the carbides precipitating on austenite grain
boundaries. Thus, grain boundaries, as compared with intragranular,
are the places where carbides tends to readily precipitate and
where carbides readily condense, so that the strength of grain
boundaries itself tends to decrease.
Therefore, it becomes possible to improve the toughness of steel
materials when coarse carbide precipitation and/or carbide
condensation at austenite grain boundaries is prevented. For such
reasons, high levels of toughness cannot be attained without
controlling the carbides precipitating on grain boundaries when the
austenite grains are relatively coarse as with the steel species
disclosed in the above-cited Japanese Patent Application Laid-open
No. S58-224116 and Japanese Patent No. 2579094.
From such viewpoints, methods of inhibiting the precipitation of
carbides which tend to become coarse at austenite grain boundaries
have recently attracted attention. Among carbides which may occur
in low alloy steel species containing Cr and Mo, there are the
types M.sub.3 C, M.sub.7 C.sub.3, M.sub.23 C.sub.6, M.sub.3 C and
MC. Among these, carbides of the M.sub.23 C.sub.6 type are
thermodynamically stable and readily precipitate and, at the same
time, are coarse carbides, so that they decrease the toughness of
steel materials. Further, M.sub.3 C type carbides are acicular in
shape and increase the stress concentration coefficient, hence
decrease the SSCC resistance.
For the reasons mentioned above, methods have now been proposed for
inhibiting the precipitation of M.sub.23 C.sub.6 type and/or
M.sub.3 C type carbides. For example, Japanese Patent Application
Laid-open No. 2000-178682, Japanese Patent Application Laid-open
No. 2000-256783, Japanese Patent Application Laid-open
No.2000-297344, Japanese Patent Application Laid-open No.
2000-17389 and Japanese Patent Application Laid-open No.2001-73086
disclose steel species or steel pipes with reduced contents of
M.sub.23 C.sub.6 type carbides. However, the methods disclosed in
these publications pay attention only to the controlling of
M.sub.23 C.sub.6 type carbides but do not take into consideration
the influences of the austenite grain size; therefore, it must be
said that the hardenability of steel is sacrificed in them.
In other words, under the circumstances, none of the methods
relying only on the technique of reducing the austenite grain size
or only on the technique of controlling carbides tending to become
coarse can accomplish the intended objects in producing steel
species or steel pipes having high strength and high toughness and
excellent sulfide stress corrosion cracking resistance (SSCC
resistance) at low cost. Therefore, guidelines are desired for
optimally combining and for making good use of both the effect of
carbide control and the effect of reducing the austenite grain size
so that steel species or steel pipes suited for use in oil well
environments can be produced at low cost.
DISCLOSURE OF INVENTION
As mentioned hereinabove, when an attempt is made to increase the
toughness only by the technique of reducing the size of austenite
grains, the hardenability of steel materials decreases. Since when
the hardenability decreases, the performance characteristics
required of steel materials cannot be secured any longer, it
becomes necessary to add an expensive element or elements to
thereby make up the decrease in hardenability and secure the
required performance characteristics. Therefore, the technique of
reducing the austenite grain size, when employed alone, results in
an increase in the content of expensive elements, hence, as a
whole, in an increase in steel material production cost.
Furthermore, even when oil well steel pipes are produced using a
steel material relatively coarse in grain size, it is difficult to
secure a required level of toughness. For securing such toughness,
it is effective to control carbides precipitating at grain
boundaries and thereby strengthen the austenite grain boundaries
themselves. However, when emphasis is placed only on the control of
the morphology of carbides without paying any attention to the
influences of the austenite grain size, the hardenability of steel
materials will lower, with the result that no high toughness can be
obtained.
Therefore, it is desired that some guidelines for optimally
combining the effect of carbide control and the effect of reducing
the size of austenite grains be provided and that oil well steel
pipes having high toughness be developed by employing the
guidelines.
It is an object of the present invention, made in view of the above
problems, to provide a highly tough steel material suited for use
in producing steel pipes to be used in oil well environments, which
are expected to be more and more severe in the future, by using the
above material as the starting material.
To accomplish the above object, the present inventors melted steel
materials having various chemical compositions, varied the
austenite grain size by varying the heat treatment conditions, and
investigated the relationship between the behavior of precipitation
of carbides at grain boundaries and the steel composition and,
further, the relationship between these and the toughness
performance.
As mentioned hereinabove, as the austenite grain size increases,
the hardenability of the steel material increases but the
precipitation of coarse carbides at austenite grain boundaries
becomes facilitated and the toughness deteriorates with the
precipitation of coarse carbides. While the toughness is improved
when the austenite grain size decreases, further detailed
investigations revealed, in addition to the above effect, that the
precipitation of coarse carbides can be prevented by reducing the
austenite grain boundaries. This is due to the increase in number
of sites where carbides readily precipitate and the resulting
dispersion of precipitation, leading to reduction in size of
individual carbides. Furthermore, regarding the characteristics of
carbides found at austenite grain boundaries, the inventors could
obtain the following findings (1) to (4). (1) Upon analysis of the
composition of carbides precipitated at austenite grain boundaries,
the main elements in the carbides were Fe, Cr, Mo and the like in
addition to C. It was also confirmed that the carbides precipitated
within granules are smaller than the carbides precipitated at
austenite grain boundaries. Therefore, the composition of carbides
precipitated within granules was examined and found that the
carbides are almost free of Mo. (2) While it is generally said that
the shape (acicular or spherical) of carbides is determined by the
tempering temperature, it was found that when the Mo content in
carbides differs, the shape of carbides varies even at the same
tempering temperature. (3) In view of the above findings (1) and
(2), the content of Mo in carbides was supposed to be a factor
exerting influences on the morphology and size of carbides, and the
composition of carbides precipitated at austenite grain boundaries
was analyzed and, as a result, it was found that the Mo content in
coarser carbides is higher and the Mo content in carbides smaller
in size is lower. In other words, by decreasing the Mo content in
carbides, it is possible to prevent the carbides precipitated at
austenite grain boundaries from becoming coarse and thereby improve
the toughness of steel materials. (4) Furthermore, as the austenite
grain size changes, the influence of the content of Mo in carbides
on the coarsening of carbides varies. Therefore, by controlling the
Mo content in carbides precipitated at grain boundaries according
to the change in austenite grain size, it is possible to adequately
prevent the precipitation of coarse carbides at austenite grain
boundaries.
The present invention, which has been completed based on the above
findings, consists in the steel materials specified below under (1)
to (4) and a method of producing steel pipes as defined below under
(5). (1) A steel material having high toughness which is
characterized in that the content of Mo [Mo] in the carbides
precipitated at austenite grain boundaries satisfies the formula
(a) given below:
where G is the austenite grain size number according to ASTM E 112.
(2) A steel material having high toughness which is characterized
in that it contains, by mass %, C: 0.17-0.32%, Si: 0.1-0.5%, Mn:
0.30-2.0%, P: not more than 0.030%, S: not more than 0.010%, Cr:
0.10-1.50%, Mo: 0.01-0.80%, sol. Al: 0.001-0.100%, B:
0.0001-0.0020% and N: not more than 0.0070% and in that the content
of Mo [Mo] further satisfies the above formula (a). (3) Desirably,
the steel material defined above under (2) further contains one or
more of Ti: 0.005-0.04%, Nb: 0.005-0.04% and V: 0.03-0.30%. (4) A
steel material having high toughness which is characterized in
that, as a more desirable chemical composition, it contains, by
mass %, C: 0.20-0.28%, Si: 0.1-0.5%, Mn: 0.35-1.4%, P: not more
than 0.015%, S: not more than 0.005%, Cr: 0.15-1.20%, Mo:
0.10-0.80%, sol. Al: 0.001-0.050%, B: 0.0001-0.0020% and N: not
more than 0.0070% and further contains one or more of Ti:
0.005-0.04%, Nb: 0.005-0.04% and V: 0.03-0.30% and in that the
content of Mo [Mo] in the carbides precipitated at austenite grain
boundaries satisfied the formula (a) given above. (5) A method of
producing highly tough steel pipes for oil wells which comprises
rolling a steel material containing the elements defined above
under (2) to (4), quenching the same from the austenite region,
wherein, after the subsequent tempering, the content of Mo [Mo] in
the carbides precipitated at austenite grain boundaries satisfies
the formula (a) given above.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a representation of the relationship between the
austenite grain size (according to ASTM E 112) and the content of
Mo (% by mass) in the carbides precipitated at austenite grain
boundaries.
BEST MODES FOR CARRYING OUT THE INVENTION
The grounds for restriction of the Mo content in the carbides
precipitated at austenite grain boundaries, the chemical
composition of the steel and the method of production as specified
above are explained below.
1. Mo Content in Carbides Precipitated at Austenite Grain
Boundaries
For providing a steel material with high toughness as well as
strength, the method is generally used which comprises reducing the
austenite grain size and conducting quenching and tempering
treatments. By reducing the austenite grain size, the impact force
exerted on individual grain boundaries is dispersed and, as a
whole, the toughness is improved. Thus, the reduction in austenite
grain size does not serve to strengthen the austenite grain
boundaries themselves but serves to reduce the grain boundary area
perpendicular to the loading of direction of the impact force to
thereby disperse the impact force and improve the toughness.
It is also possible to improve the toughness of steel materials by
strengthening the austenite grain boundaries themselves. First, the
grain boundaries can be strengthen by eliminating those elements
which segregate at grain boundaries to thereby weaken the grain
boundaries, for example P. For preventing the segregation of P, it
is required to minimize the content of P. In connection with the
cost of dephosphorization in steel making processes, steels are
saturated with a certain content level of P.
Available as other means for strengthening the austenite grain
boundaries themselves, there is the method comprising controlling
the carbides precipitated at austenite grain boundaries. The effect
of this method of grain boundary strengthening, if successful in
effectively preventing carbides from becoming coarse, may be
greater than the effect of the suppression of segregation of P in
improving the toughness of steel materials.
Therefore, in the present invention, attention was paid to the fact
that high toughness can be attained when the carbides which
otherwise occur as coarse precipitates at austenite grain
boundaries and weaken the grain boundaries are controlled. Thus,
when coarse carbides precipitate or aggregates of carbides
precipitate at austenite grain boundaries, the toughness is
deteriorated but, when relatively small carbides precipitate
dispersedly at austenite grain boundaries, the toughness is rather
improved.
Then, the inventors paid their attention to the fact that by
controlling the Mo content in the carbides precipitated at
austenite grain boundaries to an optimum level, it becomes possible
to obtain highly tough steel materials as a result. Thus, when the
Mo content in the carbides precipitated at austenite grain
boundaries is small, the coarsening of the carbides can be
prevented whereas when the Mo content in the carbides is high, the
coarsening of the carbides is promoted.
FIG. 1 shows the relationship between the austenite grain size
(according to ASTM E 112) and the Mo content (% by mass) in the
carbides precipitated at austenite grain boundaries. As the value
of the austenite grain size number G increases, the austenite grain
size decreases. The toughness characteristics are evaluated, for
example, by testing Charpy test specimens according to ASTM A 370
as to whether they have characteristics such that they show a
transition temperature of not higher than -30.degree. C. When they
satisfy the requirement that the transition temperature should be
not higher than -30.degree. C., they are evaluated as having high
toughness. In each toughness evaluation, the test is carried out
using a set of three test specimens as a unit.
As is evident from FIG. 1, high toughness regions which satisfy the
transition temperature requirement of not higher than -30.degree.
C. can be caused to appear, even when the austenite grain size is
coarse, by reducing the Mo content in the carbides precipitated at
austenite grain boundaries. This means that by reducing the Mo
content in the carbides precipitated at austenite grain boundaries,
it is possible to prevent the carbides precipitated at austenite
grain boundaries from becoming coarse or aggregating and, further,
that the critical value of the Mo content, which affects the
carbide morphology control and the toughness characteristics of
steel materials, varies depending on the austenite grain size.
From the results shown in FIG. 1, it is seen that it is necessary
for the Mo content [Mo] in carbides and the austenite grain size
number G to satisfy the relation represented by the formula (a)
given below.
The austenite grain size can be controlled mainly by selecting the
quenching conditions and can further be controlled by adding one or
more of Al, Ti and Nb. On the other hand, the factors controlling
the Mo content in carbides consist in controlling the quenching
conditions, tempering conditions and additive elements (in
particular Mo). When the quenching conditions are varied, the
degrees of redissolution and uniformity in dispersion of carbides
vary and the content of Mo in carbides varies. When the tempering
conditions are varied, the rates of diffusion of additive elements
vary and, as a result, the Mo content in carbides varies. On the
other hand, the content of Mo in carbides is greatly influenced by
the additive elements, in particular the level of addition of Mo
and other carbide-forming elements. For controlling the austenite
grain size and the Mo content in carbides, it is thus necessary to
adequately adjust the heat treatment conditions and the additive
elements.
In the practice of the present invention, the Mo content in the
carbides precipitated at austenite grain boundaries can be
determined by combining the extraction replica method with an EDX
(energy dispersive X-ray spectrometer). The "EDX" is a kind of
fluorescent X-ray analyzer and depends on an electric spectroscopic
method using a semiconductor detector.
In the present invention, the Mo content in the carbides
precipitated at austenite grain boundaries was determined by
observing austenite grain boundaries in five arbitrarily selected
views at a magnification of 2,000, selecting three large carbides
in each view and taking the mean value of the 15 values in total as
the Mo content in the carbides.
2. Chemical Composition
In the following, the chemical composition effective for the steel
material of the present invention is described. The chemical
composition referred to herein is based on percentage by mass.
C: 0.17-0.32%
C is contained for the purpose of securing the strength of the
steel material. However, when its content is less than 0.17%, the
hardenability is unsatisfactory and the required strength can
hardly be secured. For securing the hardenability, it becomes
necessary to add an expensive additive(s) in large amounts. When
its content exceeds 0.32%, hardening cracks may occur and, at the
same time, the toughness deteriorates. Therefore, the C content
should be 0.17% to 0.32%, desirable 0.20% to 0.28%.
Si: 0.1-0.5%
Si is an element effective as a deoxidizing element and at the same
time contributes to an increase in resistance to temper softening
and thus to an increase in strength. For the production of its
effect as a deoxidizing element, the content of not less than 0.1%
is necessary while, when its content exceeds 0.5%, the hot
workability becomes markedly poor. Therefore, the Si content of
0.1-0.5% was selected.
Mn: 0.30-2.0%
Mn is a component which improves the hardenability of steel and
secures the strength of steel materials. However, at a content
below 0.30%, the hardenability is insufficient and both the
strength and toughness decrease. Conversely, at a content exceeding
2.0%, the segregation in the direction of thickness of steel
materials is promoted and, accordingly, the toughness decreases.
Therefore, the Mn content should be 0.30-2.0%, desirably
0.35-1.4%.
P: Not more than 0.030%
While it is required to minimize the content of P so that the grain
boundaries may be strengthened, P is unavoidably present in steel
as an impurity. Although processes for dephosphorization have so
far been developed and improved, a prolonged time is required
therefor for reducing the P content and therefore the temperature
of the molten steel lowers, making the subsequent process operation
difficult. Therefore, it is allowed to be contained at a certain
saturation level. At a P content exceeding 0.030%, however, it
segregates at grain boundaries and causes a degrease in toughness.
Therefore, its content should be not more than 0.030%, desirably
not more than 0.015%.
S: Not more than 0.010%
S occurs unavoidably in steel and binds to Mn or Ca to form such
inclusions as MnS or CaS. These inclusions are elongated in the
step of hot rolling and thereby take an acicular shape,
facilitating stress concentration and thus adversely affecting the
toughness. Therefore, the S content should be not more than 0.01%,
desirably not more than 0.005%.
Cr: 0.10-1.50%
Cr is an element improving the hardenability and at the same time
effective in protecting carbon dioxide gas corrosion in carbon
dioxide-containing environments. However, its addition in excessive
amounts facilitates the formation of coarse carbides. Therefore,
the upper limit to its content is set at 1.50%. From the viewpoint
of preventing the formation of coarse carbides, the upper limit of
1.20% is desirable. On the other hand, for the effect of adding Cr
to be produced, the lower limit to its content is set at 0.10%,
more desirably at 0.15%.
Mo: 0.01-0.80%
Mo is effective in controlling the precipitation morphology of
carbides appearing at austenite grain boundaries and is a useful
element in obtaining highly tough steel materials. Furthermore, it
is also effective in increasing the hardenability and preventing
the grain boundary embrittlement due to P. For making it to produce
these effects, its content should be within the range of
0.01-0.80%. Amore desirable content is 0.10-0.80%.
Sol. Al: 0.001-0.100%
Al is an element necessary for deoxidation. When the content of
sol. Al is below 0.001%, insufficient deoxidation results,
deteriorating the steel quality and decreasing the toughness.
Conversely, when the content is excessive, a decrease in toughness
may rather be caused. Therefore, the upper limit is set at 0.100%,
desirably at 0.050%.
B: 0.0001-0.0020%
The addition of B can result in a marked improvement in
hardenability and, therefore, the level of addition of expensive
alloying elements can be reduced. In particular, even in the case
of producing thick-walled steel pipes, the target strength can
readily be secured by adding B. However, when its content is below
0.0001%, these effects cannot be produced and, conversely, at
levels exceeding 0.0020%, the precipitation of carbonitrides at
grain boundaries becomes easy, causing toughness deterioration.
Therefore, the B content should be 0.0001-0.0020%.
N: Not more than 0.0070%
N is unavoidably present in steel and binds to Al, Ti or Nb to form
nitrides. In particular when AlN or TiN precipitates in large
amounts, the toughness is adversely affected. Therefore, its
content should be not more than 0.0070%.
Ti: 0.005-0.04%
It is not necessary to add Ti. When added, it forms the nitride TiN
and is thus effective in preventing grain coarsening in high
temperature ranges. For attaining this effect, it is added at a
level not lower than 0.005%. However, when its content exceeds
0.04%, the amount of TiC formed upon its binding to C increases,
whereby the toughness is adversely affected. Therefore, when Ti is
added, its content should be not more than 0.04%.
Nb: 0.005-0.04%
It is not necessary to add Nb. When added, it forms the carbide and
nitride NbC and NbN and is effective in preventing grain coarsening
in high temperature ranges. For attaining this effect, it is added
at a level of not lower than 0.005%. However, at an excessive
addition level, it causes segregation and elongated grains.
Therefore, its addition level should be not more than 0.04%.
V: 0.03-0.30%
It is not always necessary to add V. When added, it forms the
carbide VC and contributes to increasing the strength of steel
materials. For attaining this effect, it is added at a level not
lower than 0.03%. However, when its content exceeds 0.30%, the
toughness is adversely affected. Therefore, its content should be
not more than 0.30%.
3. Production Method
The production method of the present invention employs the process
comprising rolling a steel material having the above chemical
composition as a base material, quenching from the austenite region
and then tempering, so that the Mo content [Mo] in the carbides
precipitated at austenite grain boundaries may satisfy the above
formula (a). The steps of quenching and tempering to be employed
here may comprise either an in-line heat treatment process or an
off-line heat treatment process.
In the in-line heat treatment process, following rolling, soaking
within the temperature range of 900.degree. C. to 1,000.degree. C.
and water quenching are carried out so that the austenitic state
may be maintained, or, after rolling, water quenching is carried
out in the austenitic state, followed by tempering under conditions
such that the steel material acquires the required strength, for
example a yield strength of about 758 MPa.
In the off-line heat treatment process, the steel pipe after
rolling is once cooled to ordinary temperature with air and then
again heated in a quenching furnace and, after soaking within the
temperature range of 900.degree. C. to 1,000.degree. C., subjected
to water quenching and thereafter to tempering under conditions
such that the steel material acquires the required strength, for
example a yield strength of about 758 MPa.
EXAMPLES
For confirming the effects of the steel materials according to the
present invention, 13 steel species specified below in Table 1 were
prepared. All the steel species satisfied the chemical composition
ranges specified hereinabove.
Billets with an outside diameter of 225 mm were produced from each
of the above steel species, heated to 1,250.degree. C. and made
into seamless steel pipes with an outside diameter of 244.5 mm and
a wall thickness of 13.8 mm by the Mannesmann mandrel method. Each
steel pipe manufactured was then subjected to an in-line or
off-line heat treatment process.
In the in-line heat treatment process, for maintaining the
austenitic state, each piper after rolling for pipe manufacture was
subjected to soaking under various temperature conditions and to
water quenching and then to 30 minutes of soaking, for tempering
treatment, at a temperature such that the steel pipe might acquire
a yield strength of about 758 MPa. Prior to quenching, the
temperature for maintaining the austenitic state was varied within
the range of 900.degree. C. to 980.degree. C. to evaluate the
effect of the austenite grain size.
In the off-line heat treatment process, after pipe-forming rolling
under the same conditions, each steel pipe was once air-cooled to
ordinary temperature, then again heated in a quenching furnace and,
after soaking under various temperature conditions, subjected to
quenching and the subsequent 30 minutes of tempering treatment at a
temperature adequate for attaining a yield strength of about 758
MPa. In the off-line heat treatment process, too, the temperature
for maintaining the austenitic state prior to quenching was varied
within the range of 900.degree. C. to 980.degree. C. For obtaining
a still finer austenite grain size, the quenching and tempering
were repeated twice.
TABLE 1 Steel species C Si Mn S P Cr Mo Ti V Nb sol. Al B N A 0.25
0.30 0.50 0.004 0.009 1.01 0.13 0.025 -- 0.025 0.026 0.0013 0.0046
B 0.26 0.29 0.50 0.002 0.018 1.02 0.50 0.022 -- 0.026 0.028 0.0010
0.0045 C 0.26 0.31 0.45 0.001 0.013 1.02 0.71 0.017 0.09 0.020
0.036 0.0015 0.0039 D 0.27 0.30 0.44 0.003 0.015 1.00 0.71 0.012 --
0.024 0.030 0.0011 0.0035 E 0.26 0.29 0.48 0.004 0.012 0.50 0.20
0.011 -- -- 0.032 0.0011 0.0051 F 0.26 0.31 0.45 0.007 0.013 0.49
0.49 0.022 -- 0.025 0.036 0.0015 0.0039 G 0.27 0.25 0.49 0.004
0.011 0.50 0.72 0.020 -- 0.024 0.038 0.0012 0.0043 H 0.23 0.30 1.32
0.006 0.023 0.20 0.70 0.010 -- -- 0.029 0.0001 0.0041 I 0.27 0.36
0.61 0.002 0.015 0.61 0.30 0.014 0.06 -- 0.032 0.0013 0.0041 J 0.20
0.46 1.48 0.006 0.020 0.56 0.10 -- -- -- 0.016 0.0002 0.0047 K 0.29
0.12 0.42 0.003 0.015 0.60 0.32 0.038 -- 0.020 0.042 0.0008 0.0040
L 0.25 0.33 0.47 0.006 0.013 1.28 0.76 0.006 0.28 0.012 0.030
0.0009 0.0058 M 0.23 0.46 0.60 0.005 0.020 1.01 0.26 -- -- 0.040
0.032 0.0001 0.0030 (The balance being Fe and unavoidable
impurities)
Curved tensile test specimens defined in the API standard, 5CT, and
full-size Charpy test specimens defined in ASTM A 370 were taken,
in the lengthwise direction, from each steel tube after the above
mentioned heat treatment process, and subjected to tensile testing
and Charpy impact testing, and the yield strength (MPa) and
fracture appearance transition temperature (.degree. C.) were
measured.
At the same time, test specimens for grain size measurement and
test specimens for microscopic observation were taken, and the
austenite grain size (grain size number defined in ASTM E 112) was
measured and the Mo content in the carbides precipitated at
austenite grain boundaries was determined by the combined use of
the extraction replica method and an EDX. The results thus obtained
are shown below in Table 2. The Charpy impact test was carried out
on the three-set unit basis.
As is evident from the results shown in Table 2, the toughness is
not affected when the austenite grain size is small, even when the
Mo content in the carbides precipitated at austenite grain
boundaries is rather high. As the austenite grain size increases,
however, the toughness deteriorates with the increase in the Mo
content in the carbides precipitated at grain boundaries. As
mentioned above, this is due to the fact that the carbides tend to
become coarse as the Mo content in the carbides precipitated at
grain boundaries increases, whereby the austenite grain boundaries
become embrittled.
The in-line heat treatment process, which is energy-saving and high
in productivity, tends to allow an increase in austenite grain size
as compared with the off-line heat treatment process. Therefore, it
is difficult to satisfy the high toughness requirement by employing
the in-line heat treatment process in the conventional methods. On
the contrary, however, by controlling the Mo content in the
carbides precipitated at austenite grain boundaries according to
the present invention, it is possible to attain high toughness even
when the in-line heat treatment process is employed.
In cases where the off-line heat treatment process is employed, it
is of course possible to attain high toughness relatively easily
even when the austenite grain size is increased to improve the
hardenability.
TABLE 2 Austenite Mo content in Yield Value of right Heat grain
carbides [Mo] Toughness strength Steel member of treatment size (%
by mass) evaluation* (Mpa) species formula (a) process Examples 3.6
4.3 G 728 J 5.25 In-line according 4.2 3.0 G 778 E 5.45 In-tine to
the 4.6 2.0 G 723 A 5.67 Off-line invention 4.8 5.2 G 750 E 5.82
In-line 5.2 3.5 G 743 I 6.22 Off-line 5.4 4.0 G 703 A 6.49 In-line
5.5 5.3 G 762 I 6.65 In-line 5.8 2.7 G 763 I 7.23 Off-line 6.5 8.9
G 733 M 9.48 In-line 6.7 2.5 G 755 E 10.47 Off-line 7.2 4.0 G 755 A
14.03 Off-line 7.2 12.4 G 721 C 14.03 Off-line 7.8 15.2 G 756 H
21.44 Off-line 8.0 21.0 G 723 K 25.09 Off-line 8.8 13.5 G 803 F
49.70 Off-line 9.2 16.0 G 791 G 71.69 Off-line 9.3 14.9 G 753 D
78.70 Off-line 10.2 13.3 G 782 B 186.27 Off-line 11.0 22.2 G 747 L
408.43 Off-line Compara- 4.3 12.3 F 789 C 5.50 In-line tive 4.5
15.2 F 791 D 5.61 Off-line examples 4.8 6.4 F 802 F 5.82 In-line
5.0 20.4 F 778 G 6.00 In-line 5.3 22.0 F 709 D 6.35 Off-line 5.7
9.6 F 751 G 7.01 In-line 7.0 13.5 N 778 F 12.39 Off-line 7.5 18.2 N
755 K 17.18 Off-line 7.8 24.5 N 789 B 21.44 Off-line 8.0 27.1 N 739
L 25.09 Off-line *Toughness evaluations were made according to the
following criteria: G: In the three-set testing, all the three sets
showed a transition temperature of not higher than -30.degree. C.
F: In the three-set testing, all the three or two sets showed a
transition temperature of not lower than -30.degree. C. N: In the
three-set testing, one set showed a transition temperature of not
lower than -30.degree. C. and the remaining two sets showed a
transition temperature of not higher than -30.degree. C.
As is evident from the results given above, the method of producing
steel pipes according to the present invention makes it possible to
produce, with high efficiency, those highly tough steel pipes for
oil wells which are to be used under oil well environments expected
to become more and more severe in the future, while satisfying the
requirements that the cost should be rationalized, the productivity
improved and energy saved.
Industrial Applicability
The steel material according to the invention and the method of
producing steel pipes using the same make it possible to
manufacture highly tough steel pipes for oil wells by rolling the
base material, tempering the same from the austenite region and
tempering the same while controlling the relationship between the
Mo content (% by mass) in the carbides precipitated at austenite
grain boundaries and the austenite grain size (according to ASTM E
112). Steel pipes suited for use under oil well environments
becoming more and more severe can thus be produced while satisfying
the requirements that the cost should be rationalized, the
productivity improved and energy saved. Therefore, the steel pipes
can be used widely as products for use in oil and gas well
drilling.
* * * * *